The manufacture of electronic circuits such as printed circuit boards and integrated circuits typically involves inspection and testing of each circuit in an array of circuit patterns formed on a substrate. Calibration of circuits may include probing and measurement of electrical characteristics of circuit components during a laser trimming operation that adjusts the electrical characteristics of the circuit. For correct test results, the tips of test probes of a probe card must be accurately aligned with contact locations or electrode pads of the circuit. Probe alignment systems use mechanical positioning equipment that adjusts the position of the substrate, the probe card, or both, to maintain accurate alignment across the array of circuit patterns.
FIG. 1 illustrates a prior-art test probing system 10, in which a substrate 12 is supported on a chuck 16 of a motorized workpiece positioning stage 20. Positioning stage 20 includes a linear positioning component, X-Y stage 22, supported on a platen 24, for movement in a horizontal plane in orthogonal directions X and Y. Positioning stage also includes a rotational positioning component, theta (θ) stage 26, supported on the X-Y stage 22 for rotation of the chuck 16 about a vertical Z axis. For reference, a Cartesian coordinate system frame of reference 30, indicates the directions X, Z, and θ (the Y direction is perpendicular to the view and is not shown in FIG. 1). A probe card carriage 34 holds a probe card 38 above the positioning stage 20 while a machine vision system 42, including a camera 44, controls the rotational (θ) and translational (X-Y) alignment of the substrate 12 to align it with probes 48 of the probe card 38. The probe card carriage 34 is supported below a motorized Z stage 50 that is actuated, after alignment of the probe card 38, to move the probe card 38 downwardly along the Z axis to press the probes 48 against the substrate 12 for testing of a circuit formed on the substrate 12. A Z-drive mechanism 56, which is supported on a stationary probe base 60, provides driving force for Z stage 50.
Because multiple copies of a circuit are typically formed on a single substrate in a regular array pattern, many known systems are controlled with an automated step-and-repeat positioning program that repetitively indexes the substrate in the X-Y plane between successive probing operations. In each probing operation, tips of the test probes are pressed against electrode pads of the circuit before performing electrical testing and/or trimming of the circuit. After testing and/or trimming, the test probes are then lifted away from the substrate before moving (stepping) the substrate to align the probes with the next circuit or the next test position on the same circuit.
Conventional alignment equipment allows the substrate to be accurately aligned with the X and Y axes by interposing a θ stage between the X-Y stage and the substrate, as shown in FIG. 1. This configuration of the θ and X-Y stages simplifies subsequent indexing of the substrate, requiring only a simple X or Y translation motion for each step, as described in the Background of the Invention section of U.S. Pat. No. 4,266,191 of Spano et al. In two other equipment designs, described in U.S. Pat. No. 4,677,474 of Sato et al. and U.S. Pat. No. 4,786,867 of Yamatsu, a second rotational positioning stage is provided for aligning the probe card with the X and Y axes of the X-Y stage, thereby enabling probe/substrate alignment to be more accurately maintained across the entire array of circuit patterns of the substrate. However, because these prior art mechanisms all include a θ stage tied to the X-Y stage, every adjustment of the θ stage requires a subsequent alignment compensation of the X-Y stage, as explained by Spano et al. at column 4, lines 16-24 of the '191 patent.
Furthermore, in systems having a θ stage supported on the X-Y stage, the mass of the θ stage adds to the inertia of the entire workpiece positioning stage. The added inertia slows movement in the X and Y directions and raises the center of mass of the workpiece positioning stage, thereby affecting positioning speed and accuracy.
The θ stage can also be a source of positioning error due to vibration and backlash that are induced in the θ stage mechanism each time the X-Y stage is actuated. Overall, the coupling of the θ stage with the workpiece positioning stage in conventional test probe alignment systems tends to reduce system throughput. Attempts to increase X-Y stage speed by minimizing the mass of the θ stage and reducing the height and/or mass of the chuck tend to increase backlash, decrease stiffness, sacrifice vibration resistance, and increase settling time of the workpiece positioning stage. Attempts to increase the resolution and accuracy of the θ stage also tend to increase the mass and height of the workpiece positioning stage. Consequently, designers of prior art systems have been forced to compromise system throughput to improve positioning accuracy, and vice versa.
The present inventor has recognized a need for an improved test probe alignment apparatus that will facilitate increased test throughput and improved probe alignment accuracy.